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Each cell is made of two parallel lead plates (electrodes), one coated with lead dioxide
(PbO2 ) and a solution of sulfuric acid electrolyte. Separators between the electrodes
eliminate the possibility of electrical short between the electrodes.  

During discharging the electrochemical energy stored in the electrodes is converted to
electrical energy.  The electrochemical reactions in the battery during discharging, results in
water and Lead Sulfate which coats both electrodes, recharging restores the situation.

While charging electrolysis of water decomposes, as a byproduct, water to its ingredients.
Oxygen gas is formed at the positive plate and Hydrogen gas at the negative plate.


Flooded Lead Acid Batteries

In flooded Batteries, also called Wet Lead Acid batteries, or Conventional Lead Acid
Batteries, the oxygen and hydrogen gases formed during charging are vented to the air. The
batteries therefore require periodic maintenance, performed by authorized personnel, in
order to check the sate of the battery and refill the missing water, if needed.
Due to hydrogen’s explosive nature the batteries should be located in separate unmanned
and well vented rooms. These batteries are generally used for UPS applications, only in large
installations, where long life and cost /performance predominate, and where maintenance
personnel is available.  


Sealed Lead Acid Battery (SLA Batteries)

In a Sealed Lead Acid Battery the possibility of gas venting to the air is eliminated. The
oxygen diffuses through the separator from the positive plate to the negative plate, where it
is recombined with the hydrogen to form water. These batteries do not require water
refilling and are therefore called also Free Maintenance Batteries.

To enable the possibility of pressure relief in the battery, mainly during overcharging, the
batteries are generally equipped with relief valves, regulated to preset pressure level. These
batteries are therefore called Valve Regulated Lead Acid (VRLA) Batteries.

SLA batteries are considered dry type, generally spill proof and leak proof even in case of
container cracks or fractures. They can be located within the UPS as any other electrical
component.

Following features of the SLA Battery cause UPS manufactures’ to prefer VRLA
technology for UPS Battery applications:  

  • Maintenance free, no water topping up is required during the whole service life of    
    the battery. No battery equalization charge is normally required.

  • Non hazardous, have no restrictions for rail, road, sea or air transportation as part of
    the UPS or as separate items.

  • Rugged and vibration resistant. Can be shipped via parcel post or fast shipping
    service.

  • Non corrosive can be used for marine applications or wet environment.

  • Can be located in any position (horizontal or vertical), enabling flexibility in UPS
    design and battery location.

  • Batteries can be located adjacent to each other or on top of each other (provided that
    weight and venting are considered). Compact packaging in the UPS enables using
    UPS internal batteries while increasing capacity and backup time.

  •  About 99% of the gasses are recombined within the battery; practically no
    hydrogen is emitted to the air. The batteries can be located in manned locations,
    where normal venting exists.

  •  The UPS can be designed to allow replacement or hot swapping of failed batteries
    by the operators, or by local service people.

  •  High rate discharge ability. As requested for UPS systems that generally provide
    high energies for relatively short times.

  • No memory effect. VRLA batteries retain their full charge in float condition during
    battery lifetime.
UPS Batteries
Sealed Lead Acid  VRLA Battery
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